This invention relates to an electronically-controlled fuel injection system for an internal combustion engine, and more particularly to a system of the kind above described which is applied to an internal combustion engine having odd numbers of cylinders so as to initiate injection of fuel at intervals of a predetermined crank angle in synchronism with the revolution of the engine.
An electronically-controlled fuel injection system for an internal combustion engine is disclosed in, for example, Japanese Patent Application Kokai (Laid-Open) No. 9757/72. According to the disclosure of this application, the quantity of fuel to be supplied to the engine to meet the operating requirement of the engine is regulated by the duration of pulses of a fuel injection pulse signal applied to solenoid-operated fuel injection valves, and the fuel injection timing is selected to be synchronous with an ignition signal generated by an ignition unit for the engine so as to effect two fuel injections in each operating cycle (two revolutions) of the engine.
However, this known system is only applicable to an internal combustion engine having even numbers of cylinders such as four, six or eight cylinders and is not directly applicable to an internal combustion engine having odd numbers of cylinders such as three, five or seven cylinders. This is because, in the case of the latter engine, it is impossible to control the fuel injection timing on the basis of the ignition signal in such a manner that two fuel injections occur at equal time intervals in each operating cycle of the engine. That is, it occurs that a fuel injection timing exists between one ignition timing and the next, and it is unable to utilize the ignition signal in that form for the determination of the fuel injection timing. Thus, a revolution sensor for sensing the angular position of revolution of the engine at intervals of 180.degree. is essentially required. However, additional mounting of such a revolution sensor on the engine is encountered with various practical problems including the problem of finding a suitable mounting space available in the narrow engine room of the vehicle, the problem of manufacturing and maintenance complexities due to the increase in the number of connection wires in the engine room, and the problem of increased costs due to the work required for mounting the sensor on the engine.